A. Field of the Invention
The present invention relates to switching spool valves.
B. Description of the Prior Art
Prime movers of the type including a double acting hydraulic cylinder and used to control the operation of a large device such as a gate valve on an oil tanker requires a switching valve that is capable of handling high pressure hydraulic fluid. Typical prior art valves employ a pilot valve that is coupled to and controlled by one or more poppet valve assemblies. Prior art pilot and poppet valve assemblies are illustrated in U.S. Pat. Nos. 3,790,127 and 3,838,710 owned by the assignee of the present invention and incorporated by reference herein.
In particular U.S. Pat. No. 3,838,710 discloses a poppet valve assembly that accomplishes essentially the same function as the invention of the application. While the prior unit functions satisfactorily, the large number of moving parts and highly accurate machine parts require substantially reduced reliability and increases the manufacturing cost of the overall system.
The prior art poppet valve systems are also bulky due to the four separate piston operated poppet valves utilized. In addition, the poppet valve system is large due to the size and capacity of the individual poppets.
Another valve that may be employed in this type of system employs a sliding spool for controlling the hydraulic circuitry. A spool design has the advantage of greatly reduced simplicity of design and umber of components. However, prior art spool valves are characterized by a large amount of fluid leakage between the spool and its housing making them unfit for a switching function. An additional shortcoming of prior art valves lie in their inability to "lock" a slave actuator in a predetermined position, requiring an additional "check" valve in each line to perform the function.
Inability to "lock" a prime mover, and unreliable operation in hydraulic circuits where substantial pressure differentials occur "across" the spool have been a problem in prior art valves. Pressure differentials "across" the spool and resultant forces on the spool have heretofore tended to freeze the spool in its cavity and prevent motion particularly after the valve has been inoperative for a considerable length of time. Efforts to overcome this difficulty have included balancing parts and adjustment of spool pressure areas. However, these approaches have generally resulted in increased leakage around the spool resulting in loss of the "locking" feature.